Conventionally, a variety of electrophotographic image forming apparatuses are designed and known as image forming apparatuses, such as copiers and printers. The image forming process includes forming an electrostatic latent image on the surface of a photoreceptor drum as an image bearer, developing the electrostatic latent image with a developer such as toner to be visualized as toner image, transferring the toner image onto a recording medium (typically paper) with a transfer device, and fixing the toner image on the recording paper with a fixing device using pressure and heat.
In the fixing device, a fixing member and a pressure member formed of opposing rollers, belts or their combinations contact each other to form a nip. A recording paper is tucked in the nip and a toner image is fixed thereon with heat and pressure.
Japanese published unexamined application No. 11-2982 discloses using a fixing belt stretched by plural roller members as a fixing member. A fixing device using the fixing belt includes a fixing belt (an endless belt) 204, plural rollers 202 and 203 stretching the fixing belt 204, a heater 201 included in one of the rollers 202 and 203, a pressure roller (member) 205, etc. (FIG. 21). The heater heats the fixing belt through the roller member. A toner image on a recording medium fed to a nip formed between the fixing belt and the pressure roller is fixed thereon at the nip with heat and pressure (a belt fixing method).
The fixing device may include a fixing member frictionally contacting the inner surface of the rotating fixing member. Japanese published unexamined application No. 4-44075 discloses a fixing device in which a heat-resistant film (fixing film) 213 is sandwiched between a ceramic heater 211 as a heating element and a pressure roller 212 as a pressure member 212 to form a fixing nip N, and a recording medium on which an unfixed toner image is formed is guided to the fixing nip N and transported with the film 213 to give a heat of the ceramic heater 211 and a pressure to the recording medium through the film 213 at the nip N for fixing the unfixed toner image on the recording medium with heat and pressure (FIG. 22). The fixing device using a film heating method can form an on-demand type apparatus using a low heat capacity member for the ceramic heater and the film. Further, the ceramic heater may be energized to be heated to have a predetermined fixing temperature only when an image is formed, and a waiting time from power on to image formable status of the image forming apparatus is short (quick start) and a power consumed for standby is very small (energy saving).
Japanese published unexamined applications Nos. 8-262903 and 10-213984 disclose a fixing device using a pressure belt method, including a rotatable heat fixing roll having an elastically deformable surface; an endless belt (a pressure belt) runnable while contacting the heat fixing roll; a belt nip unrotatably located on the inside of the endless belt, contacting the endless belt to the heat fixing roll with pressure, and passing a recording paper therebetween; and a pressure pad elastically deform the surface of the heat fixing roll. This fixing method uses a belt as the pressure member below and expands a contact area of a paper and the roll to largely improve heat conduction efficiency, which enables it to reduce energy consumption and downsize the fixing device.
However, although the fixing device disclosed in Japanese published unexamined application No. 11-2982 is more suitable for higher speed printing than the apparatus using a fixing roller, it has a limit in shortening a warm-up time (a time needed for apparatus to reach a printable temperature) and a first print time (a time from receiving a printing request to paper discharge through print preparation and print operation).
On the other hand, the fixing device disclosed in Japanese published unexamined application No. 4-44075 having low heat capacity can shorten the warm-up time and the first print time, and can downsize the apparatus at the same time. However, there are problems with the durability and belt temperature stability of the fixing device. Namely, the inner surface of the belt has insufficient abrasion resistance against the ceramic heater as a heat source, and the surface becomes rough after travelling for a long time and its friction resistance increases, resulting in unstable rotation of the belt, the fixing device increases in drive torque and a the recording medium slips, resulting in image displacement, or stress on a drive gear increases, resulting in damage to the gear (problem 1).
The fixing device using a film heating method locally heating the belt only at the nip, and when the rotating belt returns to the nip, the belt has the lowest temperature, resulting in poor fixation (particularly when the belt rotates fast) (problem 2).
Japanese published unexamined application No. 8-262903 discloses a method of using a glass fiber sheet impregnated with PTFE (PTFE-impregnated glass cloth) as a low friction sheet for a surface layer of the pressure pad to improve friction between the inner surface of the belt and a fixed member. However, the fixing device using a pressure belt method disclosed in Japanese published unexamined applications Nos. 8-262903 and 10-213984 has a large heat capacity and slow in heating, resulting in long warm-up time (problem 3).
To solve the problems 1 to 3, Japanese published unexamined application No. 2007-334205 discloses a fixing device including an almost pipe-shaped opposing member (including a metallic heat conductive material, a heating member and a support member) located on the inner surface of an endless belt and a resistance heating element such as ceramic heaters located on the inner surface of the opposing member to heat the opposing member, which enables it to heat the whole fixing belt, shorten the warm-up time and the first print time and eliminate calorie shortage when the belt rotates fast.
However, the fixing device disclosed in Japanese published unexamined application No. 2007-334205 presses a pressure roller as a pressure member toward the fixing belt to form a nip, which is supported by a metallic heat conductive material. Therefore, a nip width and a pressure at the nip are unstable.
Japanese published unexamined application No. 2010-96782 discloses a fixing device including a nip forming member such as contact members and fixing members, and a reinforcing member to hold and stabilize statuses, shapes and positions of a nip between a fixing belt and pressure roller, and a pipe-shaped support member.
The support member preferably has a predetermined cross-sectional shape to perform predetermined functions, i.e., closely contacting the fixing member to heat the fixing member and ensuring separability of a recording medium. However, it is difficult to precisely maintain the external dimensions of the support member because it is made of thin sheet metal. Uneven external dimensions of the support member cause uneven performance. e.g., the fixing member and the support member do not contact each other at the downstream side of the nip and the fixing member behaves unstably, resulting in poor separability of a recording medium.
Further, even the fixing device disclosed in Japanese published unexamined application No. 4-44075 only rotating the fixing film (belt) without using a support member has a problem of unstable form of the fixing film (belt) at the exit of the nip, resulting in poor separability of a recording medium.
For these reasons, a need exists for a fixing device capable of delivering improved separability of a recording medium.